There is known a printer, a copier and a multi-functional peripheral (MFP) using an electrophotographic process. As an exposure module (exposure unit) of each of these devices, there are known two methods called as a laser optical system (LSU: laser scan unit) and a print head (solid head). In the laser optical system, a photoconductive drum is exposed through a laser beam that carries out scanning with a polygon mirror. In the print head, a photoconductive drum is exposed through light output by a plurality of light emitting elements (emitters) such as an LED (Light Emitting Diode).
The laser optical system undesirably consumes much energy at the time of forming an image and the sound during operation is very noisy as it is necessary to rotate the polygon mirror at a high speed. As a mechanism for scanning the laser light is necessary, there is a tendency to be a large unit shape.
On the other hand, the print head can be constituted by a small-size exposure unit. The function of the small-size exposure unit is realized by using a lens, referred to as a rod lens array, for forming a non inverted image with the light emitted from the light emitting element (emitter). As there is no movable section, the small-size exposure unit is a silent exposure unit.
A print head using an organic Light Emitting Diode other than the LED is also developed. In a case of using the organic Light Emitting Diode, it is possible to collectively form the organic Light Emitting Diode on a substrate with a mask and the light emitting element (emitter) can be arranged with higher accuracy than a case of arranging the LED chips. Thus, if the organic Light Emitting Diode is used as the light emitting element (emitter), there is an advantage that an image can be formed with high accuracy.
For example, there is known an example in which a plurality of the light emitting elements (emitters) composed of the organic Light Emitting Diodes is formed on a glass substrate. If a current only for ensuring a light emission amount necessary for image formation with the foregoing structure flows, degradation is aggravated, and total light-emitting time and the light emission amount are reduced. If the light emission amount is reduced, appropriate image density cannot be obtained.